Jicama Day Length: Manipulate Light for Bigger Tubers

Jicama forms marketable storage roots best when vines receive warm conditions first, then experience short-day signals of roughly 10–12 hours of light for several weeks; in long-summer regions, growers can manipulate day length by using blackout cloth in late afternoon to create artificial nights. For wholesale-scale production, start with vigorous vegetative growth under natural long days, remove flowers to reduce seed diversion, then impose consistent short days once plants have enough canopy to feed root bulking. Target soil temperatures above 70°F, avoid nitrogen excess, and keep blackout timing exact because interrupted dark periods can delay tuber initiation. In temperate zones, light manipulation is most useful when paired with transplants, season extension, and a 5–6 month crop plan.

Quick Answer

Jicama requires short-day photoperiods of 10–12 hours of light to trigger tuber initiation. In regions with long summer days, use blackout cloth to artificially shorten daylight once vines are established. Begin shading in late afternoon to ensure uninterrupted dark periods, maintain soil temps above 70°F, and avoid excess nitrogen which promotes vine growth over root development.

Why Day Length Matters for Jicama

Jicama (Pachyrhizus erosus) is a short-day plant, meaning tuber formation is triggered when daylight drops below a critical threshold. In its native Central American range, natural day length decreases as the growing season progresses, signaling the plant to shift energy from vine growth to root storage. In temperate zones with long summer days, this signal never arrives naturally—resulting in lush vines but small or nonexistent tubers.

Research from the University of Florida's Tropical Research and Education Center confirms that jicama cultivars respond most reliably to photoperiods between 10–12 hours, with tuber initiation beginning 2–3 weeks after short-day exposure starts. The 'Agua Dulce' and 'Cristalina' cultivars are among the most photoperiod-sensitive varieties available to home growers.

Blackout Cloth Method: Step-by-Step

1. Establish vines first: Allow 8–10 weeks of vegetative growth under natural long days before imposing short days. Plants need sufficient leaf area to photosynthesize enough energy for tuber bulking.
2. Remove flower buds: Pinch off flower clusters as they appear. Flowering diverts carbohydrates away from root development and can reduce tuber size by up to 30%.
3. Install blackout cloth: Use 80–90% shade cloth or black plastic sheeting on a simple frame. Cover plants at 4:00 PM and remove at 8:00 AM to create a 16-hour dark period (8 hours daylight + 8 hours morning light = 16 hours total, but adjust based on your latitude to achieve 10–12 hours of actual light exposure).
4. Maintain consistency: Never interrupt the dark period with artificial light. Even brief light exposure during the critical dark phase can reset the plant's photoperiod clock and delay tuber initiation.
5. Continue for 8–12 weeks: Maintain short-day conditions until harvest. Tubers typically reach marketable size 4–5 months after transplanting.

Cultivar Selection by Growing Zone

Soil & Nutrition for Photoperiod Success

Soil preparation directly impacts how well jicama responds to day length manipulation. Use these guidelines:

• Soil temperature: Maintain 70–85°F for optimal tuber initiation. Use black plastic mulch to warm soil in cooler zones.
• Nitrogen management: Apply balanced fertilizer (10-10-10) at planting, then switch to low-nitrogen, high-potassium (5-10-15) once short days begin. Excess nitrogen promotes vine growth at the expense of tubers.
• Soil texture: Loose, well-drained sandy loam is ideal. Heavy clay restricts tuber expansion and increases rot risk.
• pH range: 6.0–7.0. Amend with lime if below 5.5 or sulfur if above 7.5.

Regional Growing Tips

Northern zones (USDA 5–7): Start seeds indoors 8–10 weeks before last frost. Transplant after soil reaches 65°F. Use blackout cloth from July onward when days exceed 14 hours. Season extension with row covers is essential.

Southern zones (USDA 8–11): Direct sow after last frost. Natural day length may be sufficient in fall plantings. For summer crops, blackout cloth is mandatory. In South Florida, plant in October for winter harvest when days are naturally shorter.

Frequently Asked Questions

When should I start blackout treatment for jicama?

Begin blackout treatment 8–10 weeks after transplanting, once vines have established sufficient canopy. Starting too early reduces the plant's ability to photosynthesize; starting too late may miss the critical photoperiod window for tuber initiation.

Can I grow jicama without blackout cloth?

In USDA zones 9–11 with naturally shorter fall/winter days, you may achieve tuber formation without blackout cloth by timing plantings to coincide with decreasing day length. In zones 7–8 with long summer days, blackout cloth is typically necessary for reliable tuber production.

How do I know if my jicama is forming tubers?

Signs of tuber initiation include: slight soil cracking around the base of the plant, reduced vine growth rate, and firm swellings visible at the root crown. Gently excavate one plant to check progress without disturbing the root system excessively.

What happens if I interrupt the dark period?

Even brief light exposure during the critical dark phase can disrupt the photoperiod signal and delay tuber initiation. Use automatic timers on blackout systems to ensure consistency, and check for light leaks around cloth edges.

Key Terms

• Short-day plant: A plant that initiates flowering or tuber formation when day length falls below a critical threshold, typically 10–12 hours for jicama.
• Photoperiodism: The physiological response of organisms to the length of day or night, controlling developmental processes like tuber initiation.
• Tuber initiation: The developmental stage when jicama begins forming storage roots, triggered by short-day signals and warm soil temperatures.
• Blackout cloth: Opaque material used to artificially shorten day length by blocking light during natural daylight hours.

Sources & Further Reading

• University of Florida IFAS Extension — Jicama Production Guide
• USDA Agricultural Research Service — Tropical Root Crop Research
• Penn State Extension — Season Extension Techniques for Specialty Crops
• Maynard, D.N., &> Hochmuth, G.J. (2007). Knott's Handbook for Vegetable Growers, 5th Edition. Wiley.